This invention relates to an improved process for surface modification of polyethylene terephthalate (PET) film involving corona discharge.
The treatment of films or other shaped articles made of PET using a corona discharge to improve their surface characteristics such as adhesive properties is well known. However, such treatment is often insufficient to achieve the desired improvements. Several attempts have therefore been made to improve the surface characteristics beyond those obtainable by simple corona discharge treatment.
U.S. Pat. No. 4,594,262, to Kreil et al., discloses a method for coating a polyester film base with a curable organic coating, by (1) continuously passing uncoated polyester film base through an inert atmosphere containing less than 100 ppm oxygen, while (2) exposing the film base to irradiation by an electron beam, to subject the film base to an absorbed dosage of at least 1 Mrad, and (3) applying a curable organic coating to the surface of the polyester film. The polyester film, after step (2), is substantially more adherent to organic coatings than such film which has not been so treated, and this greater adherence lasts for prolonged periods during storage at room temperature in air.
French patent No. 2,544,324 discloses a process for increasing the adherence of the surface of a shaped product made of a polyester composition. This process consists of the application of a corona discharge to this surface, which is continuously moving between a discharge electrode and another, opposite electrode. An atmosphere containing oxygen in an amount not exceeding 20% by volume is projected onto the surface to which the corona discharge is applied.